1. Field of the Invention
The present invention relates generally to fuel rods for a nuclear reactor and, more particularly, is concerned with an apparatus and method for inspecting or testing the integrity of nuclear fuel rods which have been filled with helium under pressure and sealed.
2. Description of the Prior Art
As is well known, pressurized water nuclear reactors are more efficient when operated at higher temperatures and pressures. At higher water pressures more heat per unit weight of water can be derived from the fuel elements or rods in the reactor core, because the water can reach a higher temperature and thereby absorb more energy, before reaching boiling point at that pressure.
The useful life of the fuel rods can be prolonged if the fuel rods are internally pressurized to withstand the higher external coolant or water pressures in the reactor core and prevent tube collapse. Thus, during fabrication of fuel rods, it is accepted practice to pressurize the fuel rod with an inert gas, such as helium. Typically, a gas orifice is provided in one of the end plugs of the fuel rod for this purpose. After pressurization, the orifice is welded closed to seal the fuel rod.
In order to ensure weld and fuel rod tube integrity, an accepted procedure in quality control is to inspect or test fuel rods for helium leaks. The common method in use to test for helium leaks is to load a number of fuel rods in a tray and slide the loaded tray into a closed ended vacuum chamber. After a sufficient level of vacuum is achieved, the vacuum chamber is connected in communication with a test mechanism and sampled for helium by mass spectrometry. Other similar testing methods of the prior art are disclosed in U.S. Pat. No. 3,247,706 to Rose et al, U.S. Pat. No. 4,117,333 to John, and French Patent 2,402,201 to Morin.
All of these prior art testing methods have several drawbacks. One drawback is that the testing method typically involves a sequence of manual operations which are slow and introduce risk of operator error. Another drawback is that the helium leak detecting mechanism typically used in the testing method is referenced to a zero helium standard. That means if no helium is detected in the vacuum chamber then the conclusion of the test is that there is no helium leak in the fuel rods being tested. However, no safeguard is provided to distinguish a detection of zero helium caused by an equipment malfunction from one caused by a valid "no leak" condition of the fuel rods.
Consequently, a need exists for improvements in the manner in which nuclear fuel rods are inspected and tested for helium leaks to enhance efficiency and ensure reliability of testing procedures and results.